meaning of contention slot

1. contention slot Or contention period. Minimum time a host must transmit for before it can be sure that no other hosts packet has collided with its transmission. If the maximum propagation delay from one host to any other is T, then a host that starts to transmit at time t0 may collide with a host that starts just before t0 + T. The first host will not detect the collision until time t0 + 2T. context That which surrounds, and gives meaning to, something else. In a grammar it refers to the symbols before and after the symbol under consideration. If the syntax of a symbol is independent of its context, the grammar is said to be context-free. context clash When a parser cannot tell which alternative production of a syntax applies by looking at the next input token "lexeme". E. g. given syntax C -> A | b c A -> d | b e If youre parsing non-terminal C and the next token is b, you dont know whether its the first or second alternative of C since they both can start with b. To discover whether a grammar has a context clash: For each non-terminal, N, with multiple alternatives, look at the first symbol of each alternatives right-hand side, call it s. If s is the empty string, then find the set FOLLOWERN otherwise find the set FIRST*s. If any of the sets for Ns alternatives intersect then there will be a context clash when parsing N. If the next input symbol is one of those in the intersection of two sets then you wont know which of the alternatives applies. FIRSTs is the set of symbols with which s can start, including s itself. If s is a non-terminal then FIRSTs also includes the first symbol of each alternative right-hand side of s. The * in FIRST*s means the "transitive closure" of FIRST which means keep applying FIRST to each element of the result until the result doesnt change. I. e. start with just the set R = s, then for each non-terminal x in R, add FIRSTx to R. Keep doing this until nothing new is added. We are really only interested in the terminals in FIRST*s but some definitions include the non-terminals. FOLLOWERN is the set of symbols which can come after N in a sentence. Find each occurrence of N on the right-hand side of a rule, e. g. M -> . . . | . . . N . . . | . . . If there is a symbol s immediately following N then add FIRST*s to the result again, were only interested in the terminal symbols in FIRST*s if there is no symbol after N in the alternative then add FOLLOWERM to the result i. e. if N can be the last symbol in an M then anything that can follow M can also follow N. If a grammar can generate the same sentence in multiple different ways with different parse tress then it is ambiguous. An ambiguity must start with a context clash but not all context clashes imply ambiguity. The context clash occurs when trying to parse the first token of the phrase with multiple parses - you will not be able to tell which alternative to take. To see if a context clash is also a case of ambiguity you would need to follow the alternatives involved in each context clash to see if they can generate the same complete sequence of tokens.

Related Words

contention slot |

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